1. Field of the Invention
The present invention relates to ultraviolet plastic eyewear protection in a wide range of ultraviolet wavelengths in general and more specifically to an ultraviolet active shield using a photochromic compound which has been described in my earlier invention (U.S. Pat. No. 5,581,090). The photochromic compound is any molecule in the spiropyran, spiroxazine groups or chromene derivatives.
2. Description of the Prior Art
Ultraviolet optical shields are used in many areas which include: the manufacture of sunglasses, window glazing, ultraviolet protection in working environments were ultraviolet radiation is harmful to a person's health.
Nearly all protective eyewear found in the market is designed to be static in color vision with the exception of PHOTOGRAY lenses, developed by Corning Incorporated, NY, where the color is dynamic: the optical filter is responsive to the amount of visible and UV light present in sunlight. In the following paragraphs some of the existing technology will be discussed.
1. Factors influencing vision
The eyes need protection from various environmental factors. One of the main factors in vision is the amount of light reaching the retina in the eye. It has been established that intense illumination may result in sever discomfort due to glare. An individual's ability to tolerate glare decreases markedly with age: a 30 year old becomes uncomfortable with only 65% of the glare that a 20 year old can tolerate and by the age of 40 this is reduced even more to about 40% (Bennett, The demographic variable of discomfort glare. Lighting design and applications, 1977, 7, 22-24). In addition to discomfort, there is now evidence that electromagnetic radiation can damage various parts of the eye. Damage to the retina can result from unexpectedly low levels of light particularly at the shorter wavelengths (Ham et al., Retinal sensitivity to damage from short wavelength light. Nature, 1976, 260, 153-155). Therefore protection is needed against UV rays, the intensity of which doubles with every 3500 feet of altitude (Sliney et al. Evaluation of optical radiation hazards. Appl. Optics 1973, 12, 1-24). UV emitting lasers such as Excimer (248 nm, 308 nm, 351 nm) and HeCd lasers (325 nm, 354 nm) used in medical, research and various industries are also harmful to the eye and even skin (K. Goudjil and R. Sandoval, Photochromic Ultraviolet Light Sensor and Applications, Sensor Review, Vol. 18, No. 3, 1998). Excessive exposure to UV leads to swelling of cornea resulting in the ocular disease called: Keratoconjunctivitis generally known as snowblindness. UV radiation is also responsible of the development of cataracts in humans.
Protection against infrared radiation should also be considered. Long term exposure to IR has long been reported to cause cataracts (Zigman et al, Sunlight and human cataracts. Invest. Ophtalmo. 1979, 18, 462-467). Although the levels of Solar IR intensity reaching earth surface are too low to produce an injury, it is, however, necessary to shield against higher intensity of infrared radiation particularly light engendered by intense sources such as lasers (CO2, NdYAG.). The Human eye can focus even a low-power laser beam to a tiny spot, increasing its irradiance about 100,000 times (Kevin Robinson, Laser Safety: Powerful Tools Need Powerful Protection, Photonic Spectra, October 1998, pp. 92-100).
2. Glare
Glare is caused by a relatively bright light or lights within the visual field of view which degrade vision and may result in discomfort (D. H. Brennan, Glare in Aviation, Health Physics, Vol.56, NO. 5, pp. 665-669, 1989). The visual deficit is maximal when the object of visual interest or target is close to the glare source. In aviation solar glare is reduced by sunglasses or tinted visors.
In addition to the glare from direct light there is veiling glare which is engendered by scattered or reflected light on the visors thus degrading the contrast in the visual scene. Individuals may see reflections of their own faces on their visors when in direct sunlight. The incidence of reflections should be minimized by plastic filters which allow light to travel in one direction. In general glare depends on the visual environment. Glare is dynamic in a sense that change in lighting conditions induces a variation in reflections and scattering of light (TJTP Van Den Berg, On the relation between glare and straylight, Documenta Ophtalmologica 78: pp. 177-181, 1991). Furthermore, as mentioned above, different individuals may be affected differently by various levels of lighting conditions.
3. Eyewear
For these reasons, some sort of eyeglasses, goggles or face shields is often indispensable. The utility of such eyewear depends on the extent of which it satisfactorily respond to 3 main criteria: (i) what are the optical properties of the lenses material: What is the total transmittance of the various components of radiation Ultraviolet, Visible and IR. What is their optical quality , freedom from distortion and the like. (ii) How resistant are the eyewear to fogging and (iii) how are the characteristics such as acuity, target detection and vision through field glasses affected by the goggles.
Most eyewear found in commerce is made of plastic to reduce weight and increase comfort level for the wearer. Goggle filters come in variety of colors: neutral, yellowish-neutral, yellow, yellow green, blue, rose, and purple. Some of the glasses found in the market have polarizing filters.
Commercially available glasses are intended first of all to prevent damage from electromagnetic radiation. One of the application has been to wear such goggles in snow fields and in sea where the amount of ultraviolet radiation present is almost doubled due to the properties of snow and water to reflect most of the UV rays. Another application is the protection against inadvertent laser reflection. However, most of existing eyewear does not provide adequate level of enhancing vision. This limitation is due to fact that optical density or transmission characteristics of materials used in eyewear is flat across all wavelengths. Safety is compromised by the reduction of visual awareness caused by conventional eye protection. This characteristic has been outlined in the invention of Donald D. Scott, U.S. Pat. No. 3,756,692 where a protective lens structure of variable opacity for installation in welder's face shield is disclosed. In particular Scott teaches a darkening UV material using an alkaline earth compound as the UV active component. However, in this instance, an ultraviolet reflecting means is necessary to block the whole range of ultraviolet radiation wavelengths.
In the U.S. Pat. No. 5,531,040 Gupta et al teach methods for making optical quality plastics lenses with photochromic additives. A casting resin containing photochromic additives is arranged between a lens mold and then cured. However, adjunction of ultraviolet stabilizers to the mixture to increase lifetime of the products was not mentioned. In addition, Gupta et al used a UV curing method which is known to be harmful to the photochromic compounds since UV radiation induces the formation of radicals, especially when the polymer matrix is not totally cured (monomers are present). Therefore, it is judicious to add ultraviolet stabilizers as well as anti-oxidant agents to the photochromic polymeric composition and refrain from using ultraviolet curing techniques when the mixture is not totally cured. The present invention teaches an ultraviolet active shield in a wide wavelength range with better lifetime and better stability. In addition the present invention teaches an ultraviolet shield capable of protecting the eye from a direct ultraviolet laser hit as demonstrated in the preferred embodiment.